Obtaining good limits of detection when using optical detection has been a challenge in capillary electrophoresis, due to the short path lengths engendered by the small capillary diameter. Increasing the path length of measuring radiation with a detection zone is therefore desirable. Several approaches are known in the art, such as the U-channel described in "Microfabrication of a Planar Absorbance and Fluorescence Cell for Integrated Capillary Electrophoresis Devices", Zhenhua Liang et al, Analytical Chemistry, vol. 68, no. 6, Mar. 15, 1996, the Z-channel described in European patent application 93203166.9 published May 18, 1994, or an in-channel reflection device in silicon, described in Manz et al, U.S. Pat. No. 5,599,503 issued Feb. 4, 1997.
The U-channel device of Liang et al has the disadvantage that, although it is of simple design, it is hard to manufacture since an optic fiber used to deliver light to the absorbance zone must have a dimension similar to the size of the channel. Sliding such a fiber into a hole in the device is a very difficult task.
In the design of absorbance cells in microfluid devices, it is desirable to reduce the loss of resolution due to the detector volume. Greater detector volume means lower resolution. This arises as follows. If the absorbance zone becomes very long (higher volume), any molecules that absorb the measuring radiation in the entire path length of the absorbance zone are detected. Thus, if the separation of two species of molecules being detected is less than the length of the absorbance zone the signals from the two species overlap and cannot be resolved. The detector length in a microfluid device is limited to about 200 .mu.m.
Manz et al, who apply principles known from 1942 (White, J. Opt. Soc. Am., 32 (1942) 285) attempt to solve this problem for certain devices by reflecting light off bounding surfaces of a channel, but the disclosed device is not readily manufactured in devices other than silicon and cannot be used very easily for capillary electrophoresis due to the use of the semiconductor silicon. The formation of mirror planes in glass at the sample introduction point is particularly hard to achieve. Other difficulties include alignment difficulty resulting from the use of optical fibers, and the necessity of forming holes through the plates defining the channel. It is an object of this invention to provide a microfluid device with enhanced path length, improved sensitivity, that may be manufactured without great difficulty in a wide variety of materials.